This invention relates generally to solar energy collectors and more particularly to solar energy collectors utilizing hollow fibers that facilitate the transmission of absorbed heat to the heat dissipating fluid medium used with said solar energy collectors.
Solar energy collectors (solar energy collecting units) for converting the energy of solar radiation into heat, as compared to solar cells, which utilize photoelectric effects, consist of an absorber to absorb the radiation as completely as possible, and a suitable arrangement whereby the heat is conducted from the absorber to either the heat accumulator or directly to the heat-utilizing device by means of a flowing medium (gas or liquid). At the present time, this method of utilizing solar energy is also known as photothermal conversion.
There is a difference between (1) flat or laminar solar energy collecting units and (2) concentration solar energy collecting units. Laminar collecting units are those wherein the incident solar radiation directly strikes the collecting unit, and concentrating collecting units are those wherein the solar radiation is concentrated by means of optical arrangements prior to striking the collecting unit.
The absorber, which is heated by the solar radiation, not only gives off its heat to the transporting or heat dissipation medium, but also loses heat to the ambient atmosphere. Such undesirable losses of heat occur with both concentrating collectors as well as with the laminar or flat collectors.
It is difficult to protect the side of the absorber receiving the solar radiation against heat losses. Heat-insulating arrangements which are arranged on said side of the absorber do in fact have to satisfy the condition that the radiation is able to pass, with the least possible obstruction, through the heat-insulating arrangements. Thus, the side of the absorber receiving the solar radiation should be fully transparent for solar radiation.
Heat losses are caused by heat conduction, convection, and by radiation exchange. Steps which are taken for suppressing these heat losses frequently are only concerned with one of the said types of the heat transfer, however, heat losses in many cases, are caused by more than one type of heat transfer occurring at the same time.
Heat losses of the solar energy collectors due to radiation exchange can be suppressed by various methods. Frequently, selectively reflecting layers are used as absorbers. Coatings on covering panes which are transparent for the solar radiation are likewise effective, but they reflect long-wave infrared. Single or multiple covering panes which are transparent for solar radiation but absorb long-wave infrared, are not as completely effective as the methods described above.
Heat losses due to heat conduction and convection are closely linked to one another with regard to solar energy collectors. For example, with laminar or flat collectors having a plurality of covering panes which are transparent for solar radiation, the distance between the absorber and the pane disposed thereabove, or between two panes, is from an interstice of about 15 mm without any influence on the heat-insulating properties of the arrangement. Any increase in the thickness of the gas layers results in an increase in convection.
One method frequently used for suppressing the heat conduction and convection is to house the absorber in a vessel which is transmissive for the solar radiation to the absorber and is capable of being evacuated. Below a certain pressure, the convection is reliably suppressed. If the pressure is still further reduced, a point is then reached wherein a further lowering of the pressure reduces the heat conduction.
All methods which have been previously referred to have been used in manifold combinations, both with concentrating collecting units and with laminar or flat collecting units.
By way of example, concentrating collectors have selectively reflecting absorber layers, selectively transmitting layers on the covering panes, or both, are known. The enclosing vessels are often evacuated to a greater or lesser degree.
With regards to flat collecting units, experiments have been conducted with many arrangements utilizing selectively reflecting absorber layers and having single or multiple pane coverings.
An additional source of heat loss has been observed with solar energy collectors having conventional blackened metal absorber plates. In these units, heat is removed from said blackened metal plates by means of tubes or ducts having a liquid flowing therethrough. Metal strips are located between the ducts or tubes and the absorber plates for the purpose of conducting heat to the former. Heat losses are believed to occur because the temperature of the metal strips between the tubes and absorber plates is higher than on the said tubes. The heat losses are approximately proportional to the difference between the mean absorber temperature and the external temperature. The mean absorber temperature is proportionally higher when each of the separate liquid-traversed ducts or tubes are disposed further apart from each other. When these ducts or passages are arranged closer to the absorber, smaller heat losses are then observed.
For both economical and constructional reasons, it is not always possible for the spacing between the ducts to be kept as small as desired. In the extreme case only a single wide duct or tube is still used beneath the absorber. However, since the flow in this wide or broad duct or tube should be the same under each part of the absorber, it is only with difficulty that it is possible to dispense with conduction passages.
If pipes are soldered, welded or adhesively bonded beneath a metal absorber plate, then it is prohibitive, for reasons of expense, to supply spiral tubes arranged as close as may be desired.
If absorbers are used in the "roller-bonding technique", then the distances between the ducts or tubes are not to be designed as narrow as may be desired. For reasons of strength, a sufficient distance (usually several centimeters) has to be maintained between the individual ducts or tubes.
An additional, but not less serious, disadvantage of absorbers with ducts having a heat-conducting liquid flowing therethrough is that practically all metals are subject to corrosion. Even copper tubes exhibit some corrosion.
Steel absorbers and aluminum roller-bonded absorbers are only to be operated as closed systems, and even then only with corrosion inhibitors present in the heat-dissipating water being circulated therethrough.
In view of the disadvantages associated with prior art systems, it has often been proposed to make the ducts or tubes of corrosion-resistant, organic synthetic plastic materials. A disadvantage associated with the use of said organic synthetic plastics is that they age quickly at relatively high temperatures and have only a low capacity for heat-conduction. Thus, the solar radiation energy converted on the absorber into heat is only inadequately transmitted to the heat dissipating liquid which is flowing in the tubes.
If a change-over to higher temperatures is made, which is quite possible when utilizing suitable methods for suppressing the heat losses, then most of the aforementioned absorber constructions are not capable of absorbing substantially higher pressures than atmospheric pressure. Excluded are tubular systems of small diameters consisting of metals having good strength properties.
A need therefore exists to provide an improved solar energy collecting unit which completely eliminates the aforementioned disadvantages and difficulties associated with prior art solar collecting units from practically all points of view, or, at least introduces considerable advantages when compared with that which has previously been known.